Many applications can be cited wherein it is desirable to separate and segregate the different constituent elements of a mixture from each other. In some instances this separation can be accomplished mechanically, and in others it can be accomplished chemically. There are, however, instances when neither conventional mechanical nor chemical means are appropriate or effective for this purpose. For example, nuclear waste remediation is an endeavor wherein it can be extremely difficult and dangerous to employ conventional methods for the purpose of separating the radionuclides in a waste material from its benign constituents. Other examples could also be cited.
In view of the difficulties that are encountered when using more conventional methods to isolate radionuclides from other material, efforts have recently been made to develop alternative methods and systems for the handling of such materials. One alternative has been to create a multi-species plasma from mixtures of material, such as nuclear waste, and to then separate the heavier mass ions of the radionuclides from the lighter mass ions of the benign constituents. An example of such a procedure is provided in U.S. application Ser. No. 970,548 which was filed by Ohkawa on Nov. 14, 1997, now U.S. Pat. No. 5,939,029, for an invention entitled "Nuclear Waste Separator" and which is assigned to the same assignee as the present invention.
It is known that in order to effectively separate ions of different mass from each other, it is necessary to somehow exploit a physical phenomenon to which the ions are susceptible and to which they will react differently. Plasma centrifuges are exemplary of devices which are capable of such exploitation. Specifically, in a plasma centrifuge, a plasma is swirled through the centrifuge chamber along helical paths. While traveling on these paths, the ions are subjected to centrifugal forces which tend to drive them away from their axis of rotation. More specifically, because the centrifugal forces are proportional to the mass of the individual ions on which they act, heavier ions experience greater centrifugal forces than do lighter ions. By exploiting this difference, the ions can be separated and subsequently collected according to their mass.
Using a variation on the physics of a plasma centrifuge, a plasma filter has also been disclosed which can be used to separate ions according to their mass. In the chamber of the plasma filter, this separation is accomplished by effectively confining ions which have a mass that is less than some preselected value, and collecting them at the exits of the chamber. Specifically, this confinement is to a defined volume inside the plasma filter chamber. The heavier mass ions, however, experience no such constraint in the chamber of the plasma filter. Instead, the heavier ions are forced to exit radially from the defined volume and can be collected either directly from the wall of the plasma filter chamber, or from specially designed collectors located on the wall of the chamber. A disclosure of such a device is provided in U.S. application Ser. No. 192,945 which was filed by Ohkawa on Nov. 16, 1998, now U.S. Pat. No. 6,096,220, for an invention entitled "Plasma Mass Filter" and which is assigned to the same assignee as the present invention. For the operation of either a plasma centrifuge or a plasma filter, however, it is necessary to inject a rotating plasma into the centrifuge chamber, and to maintain the rotation with an electric field that is applied perpendicular to the magnetic field.
In addition to centrifugal forces, it is also known that mass proportional forces can be generated on ions as they transit a curved path which will cause the ions to drift in a direction that is perpendicular to the action of the centrifugal forces and, thus, perpendicular to the plane of the ion's path. Specifically, it can be shown mathematically that the drift velocity, u.sub.d, of an ion having a mass, M, which is under the influence of a magnetic field, B.sub..theta., as it travels at a velocity v.sub.o along a curved path having a radius of curvature, R, can be expressed as: EQU u.sub.d =Mv.sub.o.sup.2 /eRB.sub..theta.
Since the electron thermal energy is comparable to the ion directed energy, the electrons will have a comparable, but opposite, drift velocity due to the perpendicular electron velocity. These opposite drifts can lead to charge separation and a vertical electric field, and the resulting E.times.B drifts can carry both electrons and ions radially outward. To avoid this plasma expulsion, a path must be provided to allow the more mobile electrons to neutralize the ions and avoid a charge build-up. The electrons can be collected at the wall, or along the field lines if the end plates are conducting and the path length is not too long. Alternatively, transparent conducting grids across the plasma can be used. This process results in a vertical current (j.sub.z) carried by the ions with no electric field. It is this current crossed with the magnetic field (j.sub.z XB) which balances the centrifugal force.
In order to isolate the effect of the drift velocities (u.sub.d) on the ions as they travel the curved path, it is necessary to establish B.sub..theta. such that the centrifugal forces on the ions are canceled. Thus, the ions will move along the curved path, and tend to drift in a direction that is perpendicular to the path's radius of curvature (R) at a drift velocity (u.sub.d). Where more than one type of ion is present in the plasma (with the heavier ions having a mass of M.sub.2 and a drift velocity u.sub.d2, and with the lighter ions having a mass of M.sub.1 and a drift velocity u.sub.d1), it can also be mathematically shown that the time, .tau., for the M.sub.1 ions to drift through a distance, h.sub.1, for the M.sub.2 ions to drift through a distance, h.sub.2, and for the ions to thereby separate from each other through a distance .DELTA.h will be: EQU .tau.=.DELTA.h/(u.sub.d2 -u.sub.d1)
Next, using the geometrical relationship between the arc distance(L) and the radius of curvature (R), namely; L=R.THETA., the arc angle,.THETA., traveled by an ion along the magnetic field while drifting through a distance, .DELTA.h, can then be expressed as: EQU .THETA.=eB.sub..theta..DELTA.h/(M.sub.2 -M.sub.1)v.sub.o
The point here is that, in accordance with the above expressions, a curved path of travel for ions in a multi-species plasma can be constructed which will generate vertical drift velocities for the ions. Further, because the drift velocity of an ion will be proportional to the mass of the particular ion, all ions in a multi-species plasma can be predictably separated from each other. Further, this separation will be according to their respective masses, after they have traveled a distance L along the path.
In light of the above it is an object of the present invention to provide an ion separator which can effectively separate ions of a multi-species plasma according to their respective masses. Another object of the present invention is to provide an ion separator which can effectively separate ions of a multi-species plasma without the need for active electrodes to support the plasma rotation. Yet another object of the present invention is to provide an ion separator which does not require a rotation of the multi-species plasma to be driven across the magnetic field before ions in the plasma can be separated from each other. Another object of the present invention is to provide an ion separator which can be geometrically and dimensionally configured to provide an effective separation of ions in a multi-species plasma according to their mass. Still another object of the present invention is to provide an ion separator and a method for its use which is simple and cost effective.